1. Field of the Invention
The present invention relates generally to multi-fuel engines and, more specifically, to a compression ignition engine powered at least partially by a first fuel pilot ignited by a second fuel having a lower auto-ignition temperature.
2. Discussion of the Related Art
Recent years have seen an increased demand for the use of gaseous fuels as a primary fuel source in compression ignition engines. Gaseous fuels such as propane or natural gas are considered by many to be superior to diesel fuel and the like because gaseous fuels are generally less expensive, and when used in compression ignition engines, provide equal or greater power with equal or better fuel economy, and produce significantly lower emissions. This last benefit renders gaseous fuels particularly attractive because recently enacted and pending worldwide regulations may tend to prohibit the use of diesel fuel as the primary fuel source in many engines. The attractiveness of gaseous fuels is further enhanced by the fact that existing compression ignition engine designs can be readily adapted to bum these gaseous fuels.
One drawback of gaseous fuels is that they exhibit significantly higher ignition threshold temperatures than do diesel fuel, lubricating oil, and other liquid fuels traditionally used in compression ignition engines. The compression temperature of the gas and air mixture is insufficient during operation of standard compression ignition engines for autoignition. This problem can be overcome by igniting the gaseous fuel with a spark plug or the like. It can also be overcome by injecting limited quantities of a pilot fuel, typically diesel fuel, into each combustion chamber of the engine in the presence of a homogenous gaseous fuel/air mixture. The pilot fuel ignites after injection and bums at a high enough temperature to ignite the gaseous fuel charge by homogenous charge compression ignition (HCCI). Pilot-ignited, compression ignition, gas-fueled engines are sometimes called “dual fuel” engines, particularly if they are configured to run either on diesel fuel alone or on a combination of diesel fuel and a gaseous fuel. They are often sometimes referred to as MicroPilot® engines (MicroPilot is a registered trademark of Clean Air Power, Inc. of San Diego, Calif.), particularly if the pilot fuel injectors are too small to permit the use of the engine in diesel-only mode. The typical true “dual fuel” engine uses a pilot charge of 6 to 10% of maximum fuel rate. This percentage of pilot fuel can be reduced to 1% of maximum, or even less, in a MicroPilot® engine. As applied to gas-fueled engines, the invention applies to true dual fuel engines, MicroPilot® engines, and other pilot-ignited, compression ignition, gas-fueled engines as well. It will be referred to simply as a “dual fuel engine” for the sake of convenience.
A disadvantage of dual fuel engines over spark-ignited engines is the potential generation of increased quantities of oxides of Nitrogen (NOX) resulting from sub-maximum ignition intensity of the pilot fuel charge and resultant less than optimal combustion of the pilot and gas fuel charges. The inventors theorize that less than maximum ignition intensity results from failing to time pilot fuel autoignition to at least generally occur after optimal penetration, distribution, and vaporization of the pilot fuel charge in the gas/air mixture. If autoignition (defined as the timing of initiation of pilot fuel combustion) occurs too soon after pilot fuel injection, the pilot fuel will be heavily concentrated near the injector because it has not yet time to spread throughout the combustion chamber. As a result, overly rich air/fuel mixtures are combusted near the injector, while overly lean mixtures are combusted away from the injector. Conversely, if autoignition occurs too long after pilot fuel injection, excessive pilot fuel vaporization will occur, resulting in misfire.
Moreover, premixed combustion of the pilot fuel, i.e., combustion occurring after the fuel mixes with air, provides greater ignition intensity than diffusion combustion, i.e., combustion occurring immediately upon injection into the combustion chamber and before the fuel mixes with air. Maximizing pre-mixed combustion of pilot fuel is enhanced by retarding autoignition to give the pilot fuel an opportunity to thoroughly mix with the air and form a homogeneous gas/pilot/air mixture. However, retarding autoignition timing is usually considered undesirable in diesel engine technology. In fact, it is almost universally agreed that optimum combustion in a conventional compression ignition diesel engine is achieved with the shortest possible ignition delay, and it is generally preferred that the ignition delay period should always be much shorter than the injection duration in order avoid an excessive rate of pressure rise, high peak pressure, and excessive NOX emissions. (See, e.g., SAE, Paper No. 870344, Factors That Affect BSFC and Emissions for Diesel Engines: Part II Experimental Confirmation of Concepts Presented in Part I, page 15). Conventional dual fuel engines, however, do not allow sufficient mixing time to maximize ignition intensity by igniting a pilot charge that is largely pre-mixed.
The need has therefore arisen to maximize the ignition intensity of a dual fuel charge.
HCCI offers an attractive alternative to traditional diesel engines because it has no throttling losses. Unlike in conventional compression ignition engines, combustion occurs simultaneously throughout the cylinder volume rather than as a flame front. However, heretofore, HCCI research has focused on the use of a gaseous fuel as the primary fuel. Minimum research has been done with respect to an HCCI engine having liquid fuel as the primary fuel due to difficulties associated with the HCCI combustion of liquid fuel. For instance, it is difficult to introduce a liquid fuel in a vapor state and to homogenously mix it with air. In addition, because both the primary fuel and the pilot fuel are in liquid form, both fuels will ignite at the same time unless the fuels are carefully selected to have different autoignition temperatures.
The problem of obtaining a homogenous mixture of a liquid fuel in air extends beyond HCCI engines to other systems in which it would otherwise be desirable to combust a homogenous charge of a liquid fuel and air.
The need has therefore arisen to enable practical HCCI combustion of a liquid primary fuel.
The need has additionally arisen to effectively vaporize a liquid fuel to permit the homogenous mixing of the liquid fuel with air.